This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
The initiation of casting in a twin roll caster presents significant problems, particularly when casting steel strip. On start-up it is necessary to establish a casting pool supported on the rolls. When steady state casting has been established the gap at the nip between the rolls is closed by the solidified strip, but on start-up the molten metal can fall through the gap without solidifying properly and it may then become impossible to produce a coherent strip. Previously, it has been thought necessary to introduce a dummy bar between the casting rolls on start-up so as to block the gap between the rolls while establishing the casting pool and to withdraw the dummy bar with the leading end of the solidified strip as it forms. The need to introduce a dummy bar slows the initial set up procedure preparatory to casting and this procedure must be repeated if a cast is aborted for any reason and it is necessary to restart casting. This is a particular problem when casting steel where the molten metal is at very high temperatures and the refractory components of the metal delivery system must be preheated to high temperature and brought into assembly immediately prior to casting and the molten metal poured within a very short time interval before the refractories can cool significantly. A start up procedure to initiate casting in a twin roll caster without the use of a dummy bar would enable casting to be restarted immediately after an interrupted or aborted cast without the need for extensive resetting of the caster apparatus.
Japanese Patent Publications JP 59215237A and JP 1133644A both disclose proposals for enabling start up of casting in a twin roll caster without the use of a dummy bar. Both of these proposals require an imposed gap variation during start up and a corresponding control of roll speed directed solely to providing a match between the gap and the thickness of the solidified steel shells at the nip in order to close the nip to establish a casting pool. In the proposal disclosed in JP 59215257A start up commences with a small roll gap and casting is started at relatively high roll speed to produce a strip thinner than required. A regular increase in roll gap is then imposed and the speed of the rolls is reduced in order to match an increase in strip thickness with the imposed roll gap variation. In the proposal disclosed in JP 113364andA start up commences with a relatively wide roll gap to enable flow over the rolls to be stabilised and the roll gap is then reduced to allow build up of a casting pool following which the roll gap is increased to produce a strip of the required thickness. Matching an imposed roll gap with an actual thickness of solidifying metal is extraordinarily difficult. Moreover, these proposals assume substantially parallel roll surfaces and an even gap during start up. However, when casting thin steel strip it has been found necessary to employ rolls with machined crowns. More specifically, in order to produce flat strip, the rolls must be machined with a negative crown, ie. the peripheral surface of each roll must have a smaller radius at its central part than at its ends, so that when the rolls undergo thermal expansion during casting they become generally flat so as to produce flat strip. The prior proposals involving an imposed gap control have generally not enabled successful start up with crowned rolls. The present invention provides an improved method in which the gap between the rolls during the casting start up is not imposed, but is responsive to the thickness of the metal being cast during the start up process. The invention makes it possible to use crowned rolls and also enables greater flexibility of casting speed control for optimisation of metal solidification conditions and rate of fill of the casting pool.
According to the invention there is provided a method of casting metal strip comprising:
holding a pair of chilled casting rolls in parallel relationship so as to form a nip between them and such that at least one of the rolls is moveable bodily and laterally relative to the other roll,
continuously biasing said one roll laterally toward the other roll,
setting an initial gap between the rolls at the nip which is less than the thickness of the strip to be cast,
rotating the rolls in mutually opposite directions such that the peripheral surfaces of the rolls travel downwardly at the nip between them,
pouring molten metal into the nip between the rotating rolls so as to form a casting pool of molten metal supported on the rolls above the nip and controlling the speed of rotation of the rolls so as to establish casting of a strip delivered downwardly from the nip which at the outset of casting is produced to a thickness which is greater than the initial gap between the rolls so that the initially formed strip forces said one roll bodily away from the other roll against the continuous bias to increase the gap between the rolls to accommodate the thickness of the initially cast strip, and
continuing casting to produce strip at said thickness and with the gap between the rolls increased beyond the initial gap.
Preferably, the peripheral surfaces of the rolls are negatively crowned when cold by being formed at their midparts to a radius which is less than the radius of end parts of those surfaces, the initial gap being set such that the end parts of the peripheral surfaces of rolls are spaced apart by no more than 1.5 mm.
Preferably, the initial spacing between the end parts of the rolls is in the range 0.2 to 1.4 mm.
The radial negative crown for each roll, being the difference in radius of the midpart and said end parts of the roll surface, may be in the range of 0.1 to 1.5 mm.
Preferably, said other roll is held against lateral bodily movement, said one roll is mounted on a pair of moveable roll carriers which allow said one roll to move bodily laterally of the other roll and said one roll is continuously biased laterally toward the other roll by application of biasing forces to the moveable roll carriers.
The initial gap between the rolls may be set by positioning of a stop means to limit bodily movement of said one roll toward the other. The stop means may for example be a stop which can be set to be engaged by one or both of the moveable roll carriers.
The biasing forces may be applied to the moveable roll carriers by means of biasing springs.